1. Field of Endeavor
The present disclosure relates to a method for controlling an inverter.
2. Background
In general, a power inverter, or inverter, is an electrical power converter that changes DC (Direct Current) to AC (Alternating Current) at any required voltage and frequency through PWM (Pulse Width Modulation) switching operation by being connected to a three-phase commercial AC power source to generate a desired power and supplies the power to a motor, where the motor controlled by the inverter in turn generates a torque to drive a load.
In general, a load applied by a motor-driving inverter system may be largely classified to a blower load, a hoist load, a drawbar load and a tension control load. The hoist load refers to a vertically moving mechanical system such as a crane, a hoist and an elevator, and particularly the crane and the hoist vary in work efficiency in response to a driving speed of a motor. That is, in a case a motor driving a crane is operated at a maximum speed within an allowable rated scope, the crane can be enhanced in work efficiency.
FIGS. 1a and 1b are flowcharts illustrating a method for controlling an inverter according to prior art. In a conventional inverter control, an output optimization function of the inverter is set (S110). A hoist load requires a deceleration/acceleration torque (50˜200%) of a torque necessary for a constant velocity operation, and in this case, a motor and a driving system must be able to supply a torque 150% more than a rated torque during the deceleration/acceleration time.
In the hoist load, torques of forward and backward directions of a motor are generated, where a torque direction is checked (S120) to set a control of a forward direction is set (S130), and to set a control of a backward direction is set (S140).
A motor must generate a forward direction torque and a backward direction torque for deceleration/acceleration of the system, and the motor must be able to change to a forward direction and a backward direction, which leads to necessity of setting a limit on forward and backward torque directions which is performed by the abovementioned S120˜S140.
Thereafter, torques to load are accumulated (S150), and in a case a set time has lapsed (S160), the number of torques accumulated during the set times and accumulated torques are calculated to obtain an average torque (S170). Successively, an optimal speed is calculated using the calculated average torque, the forward and backward torque limits set up at S130 and S140, and a rated speed of the inverter (S180). Furthermore, the optimal speed calculated at S180 is limited within a maximum frequency range (S190), whereby an optimal speed can be calculated. It is assumed that an entire operation of FIG. 1a is S100 of FIG. 1b. 
FIG. 1b is a flowchart illustrating a set of a final speed command of a motor. In a case an optimal speed is calculated at S100, a determination is made as to whether the optimal speed is applied as a final speed command (S210), and if the optimal speed is applied as a final speed command, the optimal speed calculated at S100 is determined as a final speed command (S220), and if the optimal speed is not applied as the final speed command, a command speed is determined as a final speed command (S230), and then a final speed is outputted (S240).
FIG. 2 is a graph illustrating a maximum torque curve of a motor speed, where ‘a’ refers to a region where a predetermined torque is lasted under a rated speed of a motor, and ‘b’ refers to a region where a predetermined torque is above the rated speed. In the figure, a torque outputtable by the motor at ‘b’ decreases in reverse proportion to an increased speed of the motor.
As explained above, a motor needs deceleration/acceleration torques during deceleration/acceleration operations, and in a case the motor keeps accelerating to excel the command speed while maintaining the deceleration/acceleration, the maximum speed is a speed capable of maintaining the deceleration/acceleration torques during the deceleration/acceleration, which can be expressed by the following Equation.(Optimal speed)=(rated speed)×(forward or backward torque limit)/(torque load)  [Equation 1]
An optimal speed or a command speed is set as a final speed command using the optimal speed calculated by the above Equation 1 as shown in FIG. 1b, and a final speed is outputted, whereby the output is optimized. However, referring to FIG. 2, this suffers from a disadvantage in that an optimal speed is determined through output optimization during an operation over a rated speed, and a predetermined speed is provided at an operation less than the rated speed, whereby the output optimization is impossible at the operation less than the rated speed.
Thus, there is a need to provide an apparatus for controlling an inverter capable of solving the aforementioned disadvantages or problems.